The present invention relates to subterranean operations. More particularly, the present invention relates to lost circulation compositions and methods of using such lost circulation compositions.
During the drilling of a well bore, a drilling fluid may be circulated down through the inside of the drill string, through the drill bit, and to the surface through the arnnulus between the walls of the well bore and the drill string. The drill string may be a drill pipe, a casing string, or any other suitable conduit. Among other things, the circulating drilling fluid lubricates the drill bit, carries drill cuttings to the surface, and balances the formation pressure exerted on the well bore. One problem associated with the drilling of a well bore may be the undesirable loss of large amounts of the drilling fluid into the subterranean formation. This problem may be referred to as “lost circulation” and the sections of the formation into which the drilling fluid may be lost may be referred to as “lost circulation zones.” The loss of drilling fluid into the formation is undesirable, inter alia, because of the expense associated with the drilling fluid lost into the formation. In addition to drilling fluids, problems with lost circulation may also be encountered with other fluids, for example, spacer fluids, completion fluids (e.g., completion brines) and workover fluids that may be circulated in a well bore. A variety of factors may be responsible for lost circulation. For example, the subterranean formation penetrated by the well bore may be highly permeable or may contain fractures or crevices therein. Furthermore, the formation may breakdown under the hydrostatic pressure applied by the fluid, thereby allowing the fluid to be lost into the formation. For instance, fractures in the subterranean formation may be created or enhanced due to the hydrostatic pressure of the drilling fluid with the resulting loss of drilling fluid into those fractures. As referred to herein, the “effective fracture gradient,” refers to the minimum hydrostatic pressure plus frictional losses that may be required to create or enhance a fracture in a potential lost circulation zone.
A number of methods have been developed to control lost circulation. One method involves the introduction of a settable composition into a problematic zone to prevent and/or reduce the flow of the drilling fluid into the lost circulation zone. However, this method may require an undesired interruption in the drilling process. Another method commonly used to control lost circulation involves the placement of lost circulation materials into the lost circulation zone. These conventional lost circulation materials may be placed into the formation, inter alia, as part of a drilling fluid or as a separate lost circulation pill in an attempt to control and/or prevent lost circulation. Conventional lost circulation materials may include graphitic carbon, ground battery casings, ground tires, ground nut shells (e.g., walnut shells, peanut shells, and almond shells), sized-calcium carbonate, petroleum coke, glass, mica, ceramics, polymeric beads, and the like. To increase the effectiveness of these conventional lost circulation materials, the particle sizes of the lost circulation materials have been optimized. For instance, petroleum coke having from about 35% to about 90% by weight particles between 10 mesh and 60 mesh has been used as a lost circulation material. Additionally, lost circulation materials have been added to drilling fluids in an attempt to increase the effective fracture gradient to allow for continued drilling and prevent lost circulation. For instance, the inclusion of lost circulation materials having between about 250 microns and about 600 microns in the drilling fluid may induce a screenout in the fracture tip that may reduce propagation or creation of fractures in the formation. These conventional methods for combating lost circulation, however, may not provide a desirable level of lost circulation control.